In general, we recommend users download/use Fiji. Fiji is Just ImageJ - it is simply a distribution of ImageJ that comes with a bunch of plugins bundled - ready for you to use out-of-the-box. If you are just getting started, we recommend downloading/using Fiji.

Would you be able to post/link the original image file here? This way we can get a better idea of your dataset. And also - we do not recommend saving images in jpeg… this format actually introduces artifacts, etc. You can read more on that here.

And here are some other helpful links that will shed some light on Segmentation - which is what you need to do to select the area of your image:

Principles page - collection of principles for the entire image analysis process, from acquisition to processing to analysis

Just go through that Segmentation workshop/slides that I posted above - that should really help you develop a workflow. For example - there is no need for the ‘clear outside’ step if all you care about is area measurements, etc.

You can also check out the TWS plugin I linked above as well (which is macro-compatible). Here is a quick-and-dirty segmentation that I got with just the default settings: